This invention relates to a fuel injection system for supplying pressurised fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine.
High-volatility fuels, such as DME, can present difficulties in the fast-acting systems that operate in a cyclic manner and at relatively high pressures, which is typical of ICE fuel injection systems, in regards of hydraulic tightness of system's elements. Such fuels evaporate at already quite low temperatures and lower pressures, making it difficult to prevent leakage since systems' valves have to effectively be gas-tight; this is combined with the requirement of reliable and durable operation of these same valves at high hydraulic pressure and flow, the latter often meaning large acting forces and contact pressures in the valve seats.
Failure to ensure a gas-tight seal in the valves of fuel injection systems leads to both external and internal leakages in the systems. External leakage usually appears at the injector nozzles and also in the dynamic seals of fuel pumps, although special means are known to effectively prevent these. Internal leakage is usually not as important to control as long as it is relatively small. However, in certain conditions even a small internal leakage may cause a loss of important functionality of the engine.
One such condition of the engine is a relatively long turned-off state following its operation at a relatively high temperature. The failure mode may be experienced in the form of comparatively long cranking time required for the engine to begin firing and start. The reason for the long cranking time there would be the fact that the intermediate volume between the high-pressure pump and the injector is partly or completely empty of liquid fuel and is instead filled with fuel vapour, so that the high-pressure pump needs to first re-fill that volume with liquid fuel before the pressure can rise well above the saturated vapour pressure at the given temperature to enable fuel injection. Generally speaking, the bigger is the intermediate volume, the longer it would take the pump to replenish it, so the so-called common rail fuel injection systems are particularly vulnerable in this respect on account of having to have the significant volume of the common rail placed between the high-pressure pump and the injectors. Since majority of low-viscosity fuel injection systems for diesel ermines are of the common rail type, we will hereinafter call the intermediate volume the common rail volume, or simply common rail.
Root cause of the common rail being devoid of liquid fuel in the particular conditions described above, can be internal fuel leakage out of the common rail either through injectors, through the high-pressure relief valve that is typically employed in such systems for safety and other reasons, or back through the high-pressure pump into the low-pressure part of the fuel injection system. There could also be other components hydraulically connected to the common rail, which may also contribute to such leakage. The leakage itself is driven by the higher fuel vapour pressure in the common rail and other parts of the fuel injection system installed close to engine, compared to the vapour pressure in the low-pressure part of the fuel injection system. Such vapour pressure difference is due to the difference in fuel temperatures, occurring, naturally as the warm engine keeps heating up the fuel close to it while the fuel tanks remain relatively cold. In such circumstances, even a minor leakage rate would be sufficient to displace much of the fuel out of the common rail, because a stopped hot engine will keep heating up its fuel for a long time.
The problem of vapour cavities in the common rail at the time of engine start is well known, and there are known ways of dealing with it. In some installations, a purge valve is used to allow re-tilling of the common rail with liquid fuel at a low pressure created by the low-pressure fuel feed system, before engine starting is attempted. Such a system is for example disclosed in U.S. Pat. No. 6,024,064. A disadvantage of that approach is at least two-fold: firstly, there must be a time delay allowed from the time there is a need to start the engine and the time the engine can actually be started, in order to let the low-pressure system's pump to purge the system; secondly, the purge valve has to be a high pressure-capable valve as it is connected to the common rail, thus such a valve would carry cost, reliability and other penalties. In other installations, electrically driven fuel feed systems with relatively high pressure are utilised, to be able to push the fuel through the still stationary high-pressure pump into the common rail and liquefy fuel vapour in it before actually beginning to crank the engine. These systems have similar disadvantages of needing a certain time for preparation for the actual engine start, and of being more expensive and complicated due to the need of developing higher feed pressure.
There is thus a need for an improved fuel injection system removing the abovementioned disadvantages.
The invention concerns a fuel injection system for supplying pressurised fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine, and comprises a low-pressure hydraulic circuit, a high-pressure fuel pump with an outlet valve for supply of pressurised fuel to a common rail, an injector for delivering fuel for combustion from the common rail to the engine. Opening and closing of the outlet valve is arranged to occur with a first frequency.
The invention is characterized in that said fuel injection system further comprises an isolating valve connected between the outlet valve of the fuel pump and the common rail, wherein said isolating valve is arranged to open and dose with a second frequency that is lower than said first frequency.
As described in the previous section, an ordinary check valve between the high-pressure pump and the common rail, such as one similar to an outlet valve of the pump, would not reliably carry out the function of sealing against fuel vapour escape back from the common rail. In the inventive fuel injection system, the use of an isolating valve whose operational frequency is lower compared to the operational frequency of the outlet valve of said pump, makes it possible to optimise its seat for gas-tight sealing, because it does not need to be designed for sealing high pressure nor for high-frequency operation.
According to one embodiment the second frequency is a low frequency having a magnitude similar to the frequency of appearance of demand for the common rail pressure to be raised to or maintained at a level typical for operation of the internal combustion engine, and where said first frequency is a relatively high frequency baying a magnitude similar to the frequency of the pumping strokes of at least one pumping element of the fuel pump.
The maximum differential pressures that the isolating valve has to be designed to seal against, may typically be only up to and slightly above the maximum fuel vapour pressure that can occur in use. In the case of DME, this would typically be around 60 bar. Also, according to the invention, the isolating valve is designed in such a way that during engine operation it is open, and it begins to close automatically in the absence of either high fuel pressure and/or fuel flow from the pump to the common rail when the engine is stopped or otherwise when the common rail pressure is commanded to be lowered. Thus, the seat of the valve does not need to be designed to resist significant fatigue and impact loading, and can therefore be made of a relatively soft material for a reliable gas-tight seal.
It is desirable to provide a solution to the problem of the prior art systems in a simple and inexpensive way, at the same time ensuring good reliability and durability. According to an aspect of the invention, a simple and automatically functioning isolating valve as described above that prevents the problem of emptying the common rail of fuel is provided, rather than trying to resolve the problem each time the engine needs to be started by means of utilising a relatively complex and expensive high-pressure and electrically-actuated purge valve, as in prior art systems. Preferably, the isolating valve is designed to react to flow conditions, readily opening with the flow from the high-pressure pump towards the common rail and slowly closing under the action of a return spring upon cessation of said flow or when the flow is reversed. This preferred embodiment does not need a reference to a third pressure source, but it is also possible to design the isolating valve in the form of a pressure-actuated valve, that would be open by the high pressure in the line between the high-pressure pump and the common rail, and closed by a return spring and a reference line pressure, such as the pressure in the low-pressure hydraulic circuit or ambient pressure. The opening pressure of said valve can be chosen to be well above the maximum fuel vapour pressure that can occur in use, and at the same time to be well below a minimum working pressure of a running engine. By these means, the cost of the system is kept relatively low, while the design and the automatic function of the isolating valve simplify the fuel injection system as a whole.
In a further embodiment according to the invention said isolating valve is a one-way valve connected by its outlet to the common rail and adapted to have its closing action slower than its opening action.
In another embodiment of the invention the maximum volumetric displacement of the stroke of a moving part of said isolating valve is bigger than a reverse flow from the common rail to the fuel pump, said reverse flow being measured between two consecutive forward flow pulses.
In a further embodiment according to the invention said isolating valve is a pressure-actuated valve and is adapted to be closed when the outlet pressure of said fuel pump is below a minimum normal common rail pressure of an operating engine, and is open in other conditions.
In another embodiment of the invention said isolating valve is electrically operated.
In a further embodiment according to the invention said isolating valve comprising valve seats where the closing and opening of said isolating valve is provided by said seats alternatively being in mechanical contact and non-contact with each other.
In another embodiment of the invention at least one seat of said isolating valve is made of a relatively resilient and non-metallic material.
In a further embodiment according to the invention a high-pressure relief valve is connected by its inlet to the outlet of the fuel pump.
In another embodiment of the invention a high-pressure relief valve is connected to the common rail, wherein a pressure-isolating valve is installed downstream of said pressure relief valve.
In a further embodiment according to the invention a pressure-isolating valve is connected between said common rail and said injector, wherein said pressure-isolating valve is able to open or close the hydraulic connection between the common rail and the injector depending on the pressure in the common rail.
Further advantages are achieved by implementing one or several of the features of the dependent claims.